Elizabeth Ricard-McCutchan

A fundamental issue of nuclear physics is the evolution of collective nuclear structure with variations in the number of protons or neutrons. Classic benchmarks of structure describe the limiting cases in the evolution from spherical to quadrupole deformed structures, yet the intermediate structures lacked a simple description and posed challenges to theoretical understanding. Recently, however, new models have been proposed, E(5) and X(5), which describe analytically nuclei at the critical point of the phase/shape transition from spherical to deformed structures.

To test these models, information is needed on both low-spin and medium-spin states of transitional nuclei. States of interest are populated in β-decay and heavy-ion fusion evaporation reactions, and measurements are carried out using γ-ray coincidence spectroscopy and the Recoil Distance Method (RDM). Results are obtained for ¹³⁰Xe, ¹⁶²Yb, and ¹⁶⁶Hf from experiments at the Yale University Wright Nuclear Structure Laboratory.

The N = 90 transitional nuclei (Nd, Sm, Gd, Dy) have been identified as close manifestations of the X(5) critical point model. The present work results in significant revisions to the level schemes of the neighboring nuclei ¹⁶²Yb (N = 92) and ¹⁶⁶Hf (N = 94). Many properties of these nuclei are now found to be reproduced by the X(5) model, establishing that phase transitional behavior in this mass region is not constrained to N = 90.

The success of the E(5) and X(5) critical point models in describing the properties of low-spin states in transitional nuclei prompted a re-investigation into a broader study of the evolution from spherical to deformed shapes in the framework of the Interacting Boson Approximation Model (IBA). Detailed IBA calculations of rare-earth isotopic chains are found to reproduce well the properties of all low-lying, positive parity states in these nuclei and the evolution of their structure is analyzed through a mapping of the isotopic chains into the IBA symmetry triangle. An extension of this approach to the Pt isotopic chain gives a simple interpretation of their structure compared to prior theoretical descriptions.